SEASONAL-CHANGES IN RED-BLOOD-CELL PARAMETERS

Seasonal changes in blood count (haematocrit and haemoglobin values) h ave been described in studies of small samples. Since Israel is charac terized by high summer temperatures, we attempted to determine whether there are circannual changes in blood count, and how they are influen ced by mean monthly temperature.In a cross-sectional study, venous blo od was drawn from 4317 male employees in industry aged 20-64 years. Bl ood count values were analysed by month. To estimate possible changes in serum osmolality, serum electrolytes were also examined. Values of haemoglobin and haematocrit in August were significantly lower than du ring the rest of the year. Circannual rhythms explained between 10% an d 57% of the variances of all the variables as assessed by cosinor rhy thmometry, although they had wide confidence intervals and most were n ot statistically significant. The lower mean haematocrit in August ref lected reduction in mean corpuscular volume (MCV) rather than a decrea se in red blood cell count (RBC). The change observed in haematocrit a nd haemoglobin in August is possibly related to heat acclimatization r ather than to endogenous sinusoidal circannual rhythms. Both plasma an d red cell volume expansion appear to occur. Possible seasonal changes should be taken into account in evaluation of blood counts.